There is considerable interest in developing nondestructive techniques for evaluating subsurface conditions. This interest is particularly strong in the field of integrated circuit (IC) manufacturing. Typically, during the manufacture of an IC package, a wafer of silicon or other semiconductor material is covered with thin film layers. It would be desirable to provide a system which is capable of nondestructively measuring the thickness of the layers applied to the semiconductor substrate.
Another technique used in manufacturing semiconductor devices is the diffusion or implantation of ions or dopants into the lattice structure of the semiconductor. There is a need for a technique for nondestructively evaluating the concentration levels as a function of depth. A suitable depth profiling technique could also be used for quantifying lattice structure defects, such as vacancies or for measuring any other parameter that varies with depth in the material. As discussed below, the methods of the subject invention satisfy the above-stated needs utilizing a thermal wave detection system.
In a thermal wave microscope, thermal features beneath the sample surface are detected and imaged by sensing the thermal waves that scatter and reflect from these features. Thermal features are those regions of an otherwise homogeneous material that exhibit variations relative to their surroundings in thermal conductivity, thermal expansion coefficient or volume specific heat. Variations in these thermal parameters can arise from changes in basic material composition or from the presence of mechanical defects such cracks, voids and delaminations. Variations in thermal parameters can also arise from changes in the crystalline order or structure or due to the presence of small concentrations of foreign ions or lattice defects in an otherwise perfect crystal. It is believed that thermoacoustic microscopy was first disclosed in applicant's prior U.S. Pat. No. 4,255,971, issued Mar. 17, 1981, which is incorporated herein by reference. In thermoacoustic microscopy, thermal waves are generated by focusing an intensity modulated localized heat source at a microscopic point. As discussed in the above cited patent, there are a variety of techniques for applying the periodic heat source to the sample, for example, an intensity modulated beam of electromagnetic radiation or particle beams.
Irradiation of a sample with an intensity modulated beam of energy results in a periodic heating of the sample and in the generation of thermal waves. These thermal waves can be measured by a variety of techniques depending on which method of detection is chosen. One method of detection involves the measurement of the oscillating temperature of the surface of the sample at the spot of localized heating. The oscillating temperature can be measured by placing the sample in a photoacoustic cell and measuring the pressure oscillations in the cell induced by the periodic heat flow from the sample to the gas in the cell. (See, "Scanning Photo-Acoustic Microscopy", Y. H. Wong, Scanned Image Microscopy, Academic Press London, 1980.) The oscillating surface temperature may also be measured with a laser traversing the gas or liquid medium in contact with the heated spot on the sample surface. This laser beam will undergo periodic deflections because of the periodic heat flow from the sample to the adjacent medium. (See, "The Mirage Effect in Photothermal Imaging", Fournier and Boccara, Scanned Image Microscopy, Academic Press London, 1980.) A third technique for measuring the oscillating surface temperature utilizes an infrared detector that measures the periodic infrared emission from the heated spot on the surface of the sample. (See, "Photothermal Radiometry for Spatial Mapping of Spectral and Material Properties", Nordal and Kanstad, Scanned Image Microscopy, Academic Press London, 1980.)
Another method for detecting thermal waves involves the measurement of the thermal displacement of the sample surface at the spot of localized heating. Techniques for carrying out the latter method include the use of a laser probe or a laser interferometer. (See, "Photo Displacement Imaging", Ameri, et al., Photoacoustic Spectroscopy Meeting, Technical Digest, Paper THA6-2, Optical Society of America, 1981).
A third methodology for detecting the thermal waves involves the measurement of acoustic signals. Acoustic waves are generated by the thermal waves in the sample because thermally induced stress-strain oscillations are set up in the heated region of the sample. These acoustic waves can be detected by a variety of techniques including a laser probe (See, "Probing Acoustic Surface Perturbations by Coherent Light", Whitman and Korpel, Applied Optics, Vol. 8, pp. 1567-1580, 1969); a laser interferometer (See, "Measurements Using Laser Probes", De La Rue, et al., Proc. IEE, Vol. 119, pp. 117-125, 1972); or with an acoustic transducer, such as a piezoelectric transducer, in acoustic contact with the sample. (See, U.S. pat. No. 4,255,971, cited above). Any of the above-described methods can be used to detect and measure thermal waves for performing thermal wave imaging and microscopy.
In addition to imaging, thermoacoustic microscopy can be used for other types of analyses. For example, thermoacoustic microscopy can be used to analyze the plate-mode resonant signature of bonded members to determine the quality of the bond therebetween. The latter technique is disclosed in applicant's copending U.S. patent application, Ser. No. 381,891, filed May 25, 1981, and now U.S. Pat. No. 4,484,820, which is incorporated herein by reference. As disclosed in the subject application, thermal wave detection can also be used for determining the thickness of a thin film layer on a substrate and for obtaining a profile of the concentration of thermal characteristics in a sample as a function of depth.
Accordingly, it is an object of the subject invention to provide a new and improved method for determining the thickness of a thin film applied to a substrate.
It is another object of the subject invention to provide a new and improved method for determining the thickness profile of a multilayer thin film structure utilizing thermal wave techniques.
It is a further object of the subject invention to provide a new and improved method for determining the thickness of the topmost layer in a multilayer structure using a thermal wave technique.
It is still another object of the subject invention to provide a method for profiling as a function of depth, a sample that has had its lattice structure locally disrupted through the diffusion or implantation of foreign ions, such as dopants.
It is still a further object of the subject invention to provide a method for profiling, as a function of depth, a sample containing imperfections in its lattice structure.
It is still another object of the subject invention to provide a method for evaluating a sample having thermal characteristics that vary as function of depth for any reason.
It is still a further object of the subject invention to provide a new and improved method for evaluating the thickness of a layer of material on a substrate wherein a thermal wave signal of the sample is compared with an expected thermal wave signal associated with a reference sample.